1. Field of the Invention
This invention relates generally to equipment for measuring the exhaust emissions of internal combustion engines, and more particularly to an apparatus for measuring the particulate or gaseous content of exhaust emissions utilizing critical flow venturi metering system.
2. Description of the Prior Art
Under present day federal regulations the exhaust emissions from motor vehicles must not exceed specified values of certain contaminates. See Section 1201, Chapter XII, Title 45 of the Code of Federal Regulations, as published in the Federal Register, Vol. 36, No. 128, Friday, July 2, 1971, at pages 12652 et seq. See also Kaufman U.S. Pat. No. 3,699,814.
The presence of such standards has made it imperative that the exhaust emissions from vehicle engines be tested and analyzed to determine the relative amount of impurities therein. Much effort has gone into the development of equipment for use in this field of exhaust sampling, and it is now known to deliver exhaust gases from an internal combustion engine at an accurately controlled flow rate through a test apparatus for purposes of determining and analyzing the relative amounts of impurities. The general scheme of such testing is to add dilution air, in carefully controlled amounts, to the exhaust gases. The admission of dilution air cools the sample, making it easier to work with. The diluted sample is then distributed to various sample storage units for subsequent chemical analysis. Naturally the admission of dilution air must be in carefuly controlled quantities in order for the test results to be repeatable and meaningful.
A system which satisfies these general requirements is described in the above identified portion of the Federal Register. However, the system described in the Federal Register suffers a number of difficulties and disadvantages, which are discussed in U.S. Pat. No. 3,699,814, to Kaufman, entitled "Gas Sampler", issued Oct. 24, 1972, and now assigned to the assignee of the present invention. The Kaufman patent, the disclosure of which is incorporated herein by reference, taught a much improved gaseous exhaust emissions sampler which replaced the troublesome constant displacement pump of prior systems with a critical flow venturi for metering the diluted exhaust emissions at a constant volume flow.
Initially, the analysis of motor vehicle exhaust emissions focused on the gaseous constituents, such as carbon monoxide and the oxides of nitrogen. However, with the increased popularity of diesel engines, present day exhaust emission analyzing equipment must now also be capable of measuring the hydrocarbon particulates of the exhaust emissions. Typically this is done by diverting a particulate containing sample through a filtration device, such as filter paper, for a predefined length of time, and then measuring the content of accumulated paticulate matter by weighing the filter paper.
For light duty diesel engines the particulate containing sample may be extracted directly from the mainstream diluted exhaust flow for accumulation on the filtration device. However, the procedure is not quite so simple for heavy duty diesel engines, due to the relatively high exhaust temperatures (nominally 350 degrees or higher). Accordingly, most present day exhaust emission measuring equipment employ a mixing duct where vehicle exhaust from the internal combustion engine under test is mixed with filtered dilution air. This lowers the temperature of the dilution sample. The diluted sample is then run through heat exchangers which further cool the sample and smooth out any temperature fluctuations which would otherwise alter the volumetric flow of the sample and destroy measurement accuracy. In order to accommodate heavy duty diesel engines and the higher exhaust temperatures produced by such engines, the above described equipment must be quite large to develop an adequate quantity of dilution air, and also require very large heat exchanging equipment and massive chillers for cooling the heat exchanging equipment. For example, in order to analyze the exhaust for a 500 cubic inch diesel engine, at a mainstream flow rate of 3000 cubic feet per minute, which may be considered typical in present day measuring equipment, a three ton heat exchanger measuring approximately 18 inches in diameter and about 5 feet in length would be required. In addition, the heat exchanger would require on the order of 50 ton refrigerated water chillers in order to maintain temperature requirements within Federal Register specifications. Naturally such systems are quite expensive and difficult to maintain.
One way of overcoming the problems associated with high temperature diesel exhaust is to use a double dilution system. Double dilution systems are known in which exhaust from the internal combustion engine under test is mixed with dilution air in a primary chamber or tunnel, and a sample from the primary tunnel is extracted and introduced into a secondary tunnel where additional dilution air is added. The double dilution system thereby provides a cooled, twice diluted sample which may then be analyzed for its pollutant content. Naturally, the double admission of dilution air must be carefully metered in order that the pollution content measurements will be repeatable and meaningful.
A common way of metering the admission of dilution air is through the use of constant flow systems which maintain the flow rate of both the mainstream flow within the primary tunnel and also the sample flow rate within the secondary tunnel to exacting tolerances. Maintaining constant flow conditions is not always easy to achieve. Fluctuations in temperature will directly afect the pressure-volume product of the gaseous constituents within the system, as provided by the physical gas laws. Hence, fluctuations in temperature will also affect the mass flow rates of the gaseous constitutents and any suspended particulates. Therefore, in order to maintain mass flow rates constant it has heretofore been necessary to employ heat exchangers for maintaining a constant temperature of the gaseous and particulate constituents within the system. As noted earlier, these heat exchangers are quite large and require massive (and expensive) chillers for proper operation. It follows that heavy duty diesel engines which produce even hotter exhaust gases, will require even larger heat exchangers and chillers, hence the constant flow technique for metering the diluted exhaust test samples may be too expensive or otherwise undesirable for some testing facilities.
As an alternative to the constant flow techniques described above it has been recognized that the need for massive heat exchangers and chillers can be largely eliminated by using proportional sampling techniques. Using a proportional sampling technique, the flow within the system is monitored and flow controls are put in place to regulate the flow, or at least measure it so that the percentage of pollutant constituents can be scaled accordingly. While offering the considerable advantage of eliminating expensive heat exchangers and chilling equipment, prior art proportional control systems require expensive pumps, suffer from air leakage due to the relatively high operating pressures which the systems require, and are relatively sluggish or unresponsive to flow rate fluctuations of periods shorter than 10 seconds.
One prior art proportional sampling system uses a variable speed pump for creating the sample flow. The speed of the pump is controlled in accordance with an analog signal electronically derived from flow rate measurements. Another prior art system employs a control valve with built-in mass flow meter for regulating the flow within the system. Such flow control valves are quite flow restrictive and hence must be operated at relatively high pressures, in order to yield the desired flow rate. At such relatively high pressures commercially available pumps are quite leaky. Furthermore, because of the relatively high pressure requirements, the pumps cannot be used in a vacuum developing configuration, since vacuum operation is limited to vacuum pressures below 15 psi in relation to atmosphere. At the flow rates required a vacuum pressure of 15 psi is insufficient. Hence, there has heretofore been no way to place the leak prone pump in the fluid circuit where it will not affect the accuracy of the measurement system.